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MG Midget and Sprite Technical - Clutch - Intermittent Problem

Still suffering from an intermittent clutch problem that first appeared when we took part in our first hill climb.

I've cracked the cylinder removal problems I was having previously and now have a new slave, hose and clevis pins along with a re-sealed master cylinder. The system has been thoroughly bled - many times! - and works fine most of the time.

From time to time, usually after a series of rapid changes, the gearbox baulks, usually going 3rd to 2nd and then won't go into any gear. Lift off the clutch completely for a second or two and it's fine again. It seems that the master cylinder is not returning quickly enough.

This problem was happening before the re-seal. When bleeding by working the Mcyl via the open pedal box I could push it in several times and get a good flow then it would decide to take ages to return to the rest position. It is happening less with the re-sealed cylinder but still occurs at awkward moments. I thought it might be due to a blockage in the back end of the master cylinder but it was completely clear.

I opted for a seal kit as a: at that point I couldn't get the cylinder out so re-sealed it in situ and b: I'd already spent quite a lot on the system as well as other bits so I went for the cheaper option. Would it be worth trying a new or stronger return spring in the cylinder or should I bite the bullet and get a complete new master cylinder?

Colin Mee

How's your free play at the master cylinder when the clutch is at rest? I'm a little rusty on later master cylinders, but on some if the master cylinder piston doesn't return all the way home then they can build up pressure internally (though that doesn't quite sound like what you're experiencing)

It *could* be a weak return spring. It could also be the piston catching on the bore, but I'd expect that sort of interference to be pretty obvious when you removed the piston the first time. Could it be that the bore has worn oval? That could cause occasional jamming.

Slightly "outside the box" with this thought, but maybe the splines on the gearbox input shaft, or on the clutch driven plate are worn. A wear ridge there could prevent the plate from releasing smoothly every time, causing your symptoms.
Guy W

Thanks for the comments.

Guy - I like 'outside the box' or 'off the wall' - and I'm also very good at overlooking the obvious - hence the thread! I've been wondering whether part of the problem could be the plate sticking on the splines however, the slow returning master cylinder happens when bleeding when there is not enough pressure to be operate the release mechanism.

Growler, your suggestion of a worn or oval bore makes sense. When it's working correctly there's very little play in the system although the biting point is right at the bottom of the pedal travel. While the piston's movement felt OK when it was on the bench I wouldn't describe its as perfect - I put that down to stiff new seals (they had been well soaked in fluid beforehand). Also the shiny surface didn't extend all the way round the piston as some areas were smooth black so perhaps something isn't as round as it should be. No sign of pitting or rust anywhere.
Colin Mee

I have had exactly the same problem on a race midget.
It turned out to be where the roller release bearing sits in the clutch fork. The bearing has a lug on each side which is held in place by two clips, but the clutch fork corresponding indents to accept the bearing had worn causing when actuated against the clutch fingers to pivot out of square causing the clutch to catch sometimes and not operate at other times.
D Stanton

This thread was discussed between 02/06/2013 and 04/06/2013

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